Stereotactic radiosurgery (SRS) is used either as definitive treatment alone or as a "boost" in combination with whole-brain radiotherapy (WBRT) for the treatment of brain metastases. To establish a target for SRS, the gross tumor volume (GTV) is typically defined as the contrast-enhancing volume on T1-weighted axial MRI images. In turn, the volume chosen for treatment (the planning target volume or PTV) is generated by expanding the GTV to account for microscopic extension of tumor beyond the MRI-visualized volume and deviations in patient positioning at the time of treatment., However, the optimal PTV in patients undergoing SRS of brain metastases has not been established.